Lecture 28 Point-group symmetry I

Molecular symmetry A typical conversation between chemists … Symmetry is the “language” all chemists use every day (besides English and mathematics). Formaldehyde is C 2v. The A 1 to B 2 transition is optically allowed. This vibrational mode is A g. It is Raman active.

Molecular symmetry We will learn how to classify a molecule to a symmetry group, characterize molecules’ orbitals, vibrations, etc. according to symmetry species (irreducible representations or “irreps”), use these to label states, understand selection rules of spectroscopies and chemical reactions.

Molecular symmetry We do not need to memorize all symmetry groups or symmetry species (but we must know common symmetry groups, C 1, C s, C i, C 2, C 2v, C 2h, D 2h, C ∞v, D ∞h, and all five symmetry operations/elements), memorize all the character tables, memorize the symmetry flowchart or pattern matching table, know the underlying mathematics (but we must have the operational understanding and be able to apply the theory routinely).

Mathematics behind this The symmetry theory we learn here is concerned with the point-group symmetry, symmetry of molecules (finite-sized objects). There are other symmetry theories, space- group symmetry for crystals and line-group symmetry for crystalline polymers. These are all based on a branch of mathematics called group theory.

Primary benefit of symmetry to chemistry

Symmetry logic Symmetry works in stages. (1) List all the symmetry elements of a molecule (e.g., water has mirror plane symmetry); (2) Identify the symmetry group of the molecule (water is C 2v ); (3) Assign the molecule’s orbitals, vibrational modes, etc. to the symmetry species or irreducible representations (irreps) of the symmetry group. In this lecture, we learn the symmetry elements and symmetry groups.

Five symmetry operations and elements Identity (the operation); E (the element) n-fold rotation (the operation); C n, n-fold rotation axis (the element) Reflection (the operation); σ, mirror plane (the element) Inversion (the operation); i, center of inversion (the element) n-fold improper rotation (the operation); S n, n-fold improper rotation axis (the element)

Identity, E is no operation (doing nothing), which leaves the molecule unchanged. Any and every molecule has this symmetry element.

n-fold rotation, C n Rotation through 360º/n around the axis. The axis with the greatest value of n is called the principal axis.

Reflection σ v parallel (vertical) to the principal axis σ h perpendicular (horizontal) σ d bisects the angle between two C 2 axes (diagonal or dihedral)

Inversion Inversion maps (x, y, z) to (–x, –y, –z).

n-fold improper rotation Rotation through 360º/n around the axis followed by a reflection through σ h.

Symmetry classification of molecules Molecules are classified into symmetry groups. The classification immediately informs us of the polarity and chirality of the molecule We have two naming conventions – Schoenflies and Hermann–Mauguin system (International system) – we use the former.

C 1 group has only identity symmetry element.

C i group has identity and inversion only.

C s group has identity and mirror plane only.

C n group has identity and n-fold rotation only.

C nv group has identity, n-fold rotation, and σ v only.

C nh group has identity, n-fold rotation, and σ h (which sometimes imply inversion).

D n group has identity, n-fold principal axis, and n twofold axes perpendicular to C n.

D nh group has identity, n-fold principal rotation, and n twofold axes perpendicular to C n, and σ h.

D nd group has identity, n-fold principal rotation, and n twofold axes perpendicular to C n, and σ d.

S n group molecules that have not been classified so far and have an S n axis

Cubic group Tetrahedral group: CH 4 (T d ), etc. Octahedral group: SF 6 (O h ), etc. Icosahedral group: C 60 (I h ), etc.

Flow chart Linear? Inversion? D∞hC∞h Very high symmetry? Inversion? C5?IhOhTdCn? nC2 normal to principal Cn? σh?Dnhnσd?DndDnσh?Cnhnσv?CnvS2n?S2nCnσ?Cs inversion? CiC1 YESNO YESNO YESNO YESNO YESNO YESNO YESNO YESNO YESNOYESNO YESNO YESNO YESNO YESNO

Flow chart Linear? Inversion? D∞hC∞h Very high symmetry? Inversion? C5?IhOhTdCn? nC2 normal to principal Cn? σh?Dnhnσd?DndDnσh?Cnhnσv?CnvS2n?S2nCnσ?Cs inversion? CiC1 YESNO YESNO YESNO YESNO YESNO YESNO YESNO YESNO YESNOYESNO YESNO YESNO YESNO YESNO

Pattern matching

Polarity Dipole moment should be along C n axis. There should be no operation that turn this dipole upside down for it not to vanish. Only C 1, C n, C nv, and C s can have a permanent dipole moment.

Chirality A chiral molecule is the one that cannot be superimposed by its mirror image (optical activity) A molecule that can be superimposed by rotation after reflection (S n ) cannot be chiral. Note that σ = S 1 and i = S 2. Only C n and D n are chiral.

Homework challenge #9 Why does the reversal of left and right occur in a mirror image, whereas the reversal of the top and bottom does not? Public domain image from Wikipedia

Summary We have learned five symmetry operations and symmetry elements. We have learned how to classify a molecule to the symmetry group by listing all its symmetry elements as the first step of symmetry usage. From this step alone, we can tell whether the molecule is polar and/or chiral.